When an energy-storable phosphor (e.g., stimulable phosphor, which gives off stimulated emission) is exposed to radiation such as X-rays, it absorbs and stores a portion of energy of the radiation. The phosphor then produces stimulated emission according to the level of the stored energy when exposed to electromagnetic wave such as visible or infrared light (i.e., stimulating light). A radiation image recording and reproducing method utilizing the energy-storable phosphor has been widely employed in practice. In that method, a radiation image storage panel, which is a sheet comprising the energy-storable phosphor, is used. The method comprises the steps of; exposing the storage panel to radiation having passed through an object or having radiated from an object, so that radiation image of the object is temporarily recorded in the storage panel; sequentially scanning the storage panel with a stimulating light such as a laser beam to emit a stimulated light; and photoelectrically detecting the emitted light to obtain electric image signals. The storage panel thus processed is then subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as energy-storable phosphor sheet) has a basic structure comprising a support and a phosphor layer provided thereon. However, if the phosphor layer is self-supporting, the support may be omitted. Further, a protective layer is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical damage.
Various kinds of phosphor layers are known and used. For example, a phosphor layer comprising a binder and an energy-storable phosphor dispersed therein is generally used, and a phosphor layer comprising agglomerate of an energy-storable phosphor without binder is also known. The latter layer can be formed by a gas phase-accumulation method or by a firing method.
The radiation image recording and reproducing method (or radiation image forming method) has various advantages as described above. It is still desired that the radiation image storage panel used in the method have as high sensitivity as possible and, at the same time, give a reproduced radiation image of high quality (in regard to sharpness and graininess).
In order to improve the sensitivity and the image quality, it has been proposed that the phosphor layer of the radiation image storage panel be prepared by a gas phase-accumulation method such as vacuum vapor deposition, sputtering or chemical vapor deposition (CVD). The process of vacuum vapor deposition, for example, comprises the steps of: heating to vaporize an evaporation source comprising a phosphor or starting materials thereof by means of a resistance heater or an electron beam, and depositing and accumulating the vapor on a substrate such as a metal sheet to form a layer of the phosphor in the form of columnar crystals.
The phosphor layer formed by a gas phase-accumulation method contains no binder and consists essentially of phosphor, and there are gaps among the columnar crystals of the phosphor. Because of the presence of gaps in the phosphor layer, the stimulating light can stimulate the phosphor efficiently and the emitted light can be collected efficiently. Accordingly, a radiation image storage panel having such phosphor layer has high sensitivity. Further, since the gaps in the phosphor layer prevent the stimulating light from diffusing parallel to the phosphor layer, the radiation image storage panel can give a reproduced radiation image of high sharpness.
Japanese Patent Provisional Publication 2003-050298 discloses a process for preparation of a radiation image storage panel by gas phase-accumulation. In the disclosed process, a layer of a phosphor matrix compound in the form of columnar crystals is first formed on a support by gas phase-accumulation utilizing an electron beam under a high vacuum such as 1×10−3 Pa, and then a phosphor layer comprising the matrix compound and an activator is formed thereon (so that each columnar crystal of the phosphor may be one-to-one grown on each columnar crystal of the lower phosphor matrix layer) by gas phase-accumulation utilizing an electron beam under a high vacuum condition such as 1×10−3 Pa. Thus prepared phosphor layer is improved in columnar crystallinity.
The inventors of the present invention have studied the process described in the above-mentioned Japanese Patent Provisional Publication, and found that the phosphor layer which is firmly combined with the underlayer easily separates from the support, because the underlayer (i.e., the first formed matrix compound layer) shows no function to relax stress applied to the radiation image storage panel.